Once upon a time, to change your album cover you had to start organizing it, hover on a picture and wait for a tiny blue ribbon to appear, then click it.
Now: not anymore.
Do not waste your time, you can hover on pictures for days without ever seeing the magical blue ribbon.
The new procedure is:
1) open the album
2) click on "organize"
3) click the "large thumbnails" icon
4) hover on the picture you want to promote, then click on the "Album cover" button that will appear: